Method of preparing an isocyanate



Patented Jan. 1, 1952 UNITED STATES PATENT OFFICE 7 METHOD or PREPARING AN ISOCYANATE Frederic 0. Schaefer and Erhart K. Drechsel, Stamford, Conn., assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine t No Drawing. Application August 13, 1949, Serial No. 110,224

9 Claims. (01. 260-453) 1 l y 2 This invention relates to the preparation of in accordance with the mechanism indicated isocyanates (isocyanic esters), and more particubelow: larly to a new and improved method of preparing R aliphatic hydrocarbon isocyanates, specifically alkyl and cycloalkyl isocyanates, alkenyl and 5 cycloalkenyl isocyanates, aralkyl isocyanates and 0:0 0:0 0 aralkenyl isocyanates. The invention is espe E {L Hm l cially concerned with the preparation of such isocyanates which comprises pyrolyzing the corre- I sponding tri-(aliphatic hydrocarbon) isocyanu- (L rate under anhydrous conditions in the presence of a strong, non-oxidizing acid, more particularly a hydrogen halide and specifically hydrogen chloride; at a temperature of at least 120 C., preferably at least about 170-180 (1., but below the 1 0 temperature of decomposition of the isocyanate which is formed, e.g., about 600 C., and isolating the said isocyanate, e. g., by distillation and condensation of the distillate. O

It was known prior to our invention that alig phatic esters of isocyanic acid could be prepared by reaction between an aliphatic sulfate and potassium cyanate, as well as by phosgenation of an amine corresponding to the desired iso- V R cyanate. It also has been suggested that such esters could be prepared by 'pyrolyzing an N -alkyl, N N-alkox'yalkyl or N -alkoxyalkoxyalkyl carbamate, followed by separation of the isocyanic ester from the pyrolysis products before the latter have a had time to reunite and form the starting N-sub- H & stituted carbamate. f l

'I It is a primary object of the present invention a to provide a new and improved method or process 01' preparing aliphatic hydrocarbon isocyanates. g Another object of the invention is to prepare such isocyanates by a relatively simple and inexpensive t l process from the corresponding isocyanurates. l A further object of the invention is to obtain more valuable products from a by-product "of otherehemical reactions and which has only limited utility as such.

' These and other objects are accomplished in practicing this invention by pyrolysis of an isocyanurate corresponding to the aliphatic isocyanate desired, as briefly described in the first paragraph of this specification and more-fully hereafter. The reaction is believed to proceed RNCO RNCO RNHCOG] 4o or I 2RNco RNEIC 001 In the above formulas R represents an aliphatic hydrocarbon radical including alkyl (e. g., methyl,

ethyl, propyl, isopropyl, n-butyl, isobutyl, sectbutyl, tert.-butyl, amyl; isoamyl, hexyl, heptyl,

octyl, nonyl, decyl, dodecyl, tetradecyl, hexa hydrocarbon radical containing from 1 to about 10 carbon atoms, inclusive.

The isocyanurates which are subjected to pyrolsis in accordance with thepresent invention can be prepared by methods v such as .are described in the literature; or, if desired, they can be produced by the method disclosed and claimed in the copending application of Donald W. Kaiser and Dagfrid Holm-Hansen Church,

Serial No. 106,976, filed July 26, 1949, now Patent,

No. 2,536,849, dated January 2, 1951.

The pyrolysis ,of the tri-(aliphatichydrocarbonfis ocyanurate, e. g., triallyl isocyanurate, tri-- n-butyl isocyanurate, tricyclohexyl isocyanurate, tribenzyl cyanurate, tricinnamyl isocyanurate, etc., under anhydrous conditions in the presence of (i. e,, in contact with) hydrogen ,chloride can be effected in any desired manner. For example, an organicsolvent solution or dispersion of the tri-(aliphatic hydrocarbon) isocyanurate corresponding to the isocyanate to be produced can first be saturated with hydrogen chloride by passing dry HCl gas into the dry organic solvent, which latter-is inert to the isocyanurate and to its pyrolysis products.

Illustrative examples of organic solvents that thus can be used are benzene, toluene, xylene, dioxane, ethers such as diisopropyl ether, dibutyl ether, etc., esters such as butyl acetate, etc., chlorinated hydrocarbons, for instance carbon tetrachloride, trichloroethylene, ethylene dichloride, chlorobenzenes such as 1,3-dich1orobenzene, etc., ketones such as methyl ethyl ketone, etc., petroleum naphtha, etc.

The organic solvent solution or dispersion of the isocyanurate is preferably maintained at a relatively low. temperature of the order of 10-30 C. while passing the dry HCl gas therein in order to saturate it. The saturated solution or dispersion containing absorbed hydrogen chloride is distilled ,(e. g., at atmospheric pressure) until the solvent has been removed, and the residue containing absorbed hydrogen chloride is then heated .at atemperature of at least 120 C. but below the decomposition temperature of the isocynate which is formed, e. g., about 500 to 600 C. The isocyanate is isolated from the heated residue by distillation and is condensed and collected in a suitable receiver. The carbamyl chloride by-product of the pyrolysis which does not decompose during the distillation also can be isolated, and can be converted into the corresponding isocyanate by treatment with a dehydrohalogenating agent, thereby increasing .the overallyield of isocyanate-from the starting'isocyanurate.

Alternatively, the aliphatic hydrocarbon isocyanate =.can be prepared ;by passing dry hydrois .not. precluded. can be effected in the presence of inert liquid gen chloride gas into the corresponding tri- (aliphatic hydrocarbon) isocyanurate (alone or in solution or dispersion in an inert organic solvent) while heating the latter at a temperature of at least 120 C. but below the temperature of decomposition of the isocyanate which is formed, e. g., at a temperature not higher than about 600 C., and recovering the isocyanate, e. g., by fractional condensation of the evolved pyrolysis products.

The pyrolysis of the isocyanurate is preferably effected while it is in a liquid phase, although pyrolysis of the isocyanurate in a vapor phase In either .case, the pyrolysis solvents (e. g., benzene, toluene, xylene, etc.) or

,gaseous diluents (e. g., nitrogen, carbon dioxide,

helium, argon, etc.) or other adjuvants or cata- :lystsfor thereaction (e. g., chlorine, etc.).

The amount of hydrogen chloride employed .may bervaried as .desired or as conditions may 7 moles 0f1 HC1 per mole of isocyanurate, a lower percentage conversion of the isocyanurate to the isocyanateresults.

,,As has been indicated hereinbefore, Lthe ,pye lolysis temperatures can -,be considerably varied with inthe range .of 120 C. to about l6pp? C. ,or more, the lower temperatures generally re,- quiring-a .longerlheating period. In the preparation of ,allyl isocyanateby pyrolysis of :trip allyl isocyanurate,optimum=resultshave been ob.- tained by heating the latter in the, presence pt hydrogen chloride at .a' temperature the rangeof 180 C. to 2"I 0 C.,..more particularly 18.0-.2 20 C.

The pyrolysis "F 311 he efiected at atmospheflQ, subatmospheric tor. superatmosphericpressure, lby batch, :semi continuous or continuous methods, and ,in any suitable apparatus. For example, pyrolysis can be effected in ,a vertical orihorifzontal tubularconyerter into which lthehydrogen chloride and the isocyanuratearecharged-either continuously .or intermittently and either separately-gnadmixed. ,Theconver-ter maybe-made 91 n suit le in t m a su h as cl Q t nsrtmet lem t e k l d si ed. he on e te can b cke with sui ab pack n ma erials l H QEF 1 9 elumi eenpmei-n1 lassead Bas i l nes. e c- BA suit le mean ma be s i lq e demenarati -the lisoc n t fr mlother pr ducts-of th p ro s s E9 xam e -in-- 0me=cases;i i more satisfactory to separate the isocyanate :by Inactional condensation of the evolvedpyrolysiszproducts, w ile ,inp h rs t pyrol s s products may becollecte .-in*a.scz1vent which dissolves the so- Qyanat tolform e .sep r telpha ehis phas is then withdrawn-and the isocyanate *recovered therefrom, as by distillation. mhegpyrolysis products. also -:can be condensed and collected :as ;a single fraction, and this fraction "then distilled to isolate the :isocyanate from-the other components. if desired; such distillation :can :be letfected while the pyrolysis products {are admixed with -;an inert diluent :which forms an :azeotro'pic mixture .either with the isocyanateor with the other decomposition products, thereby facilitating the'removal of the'isocyanate-irom thepyrolysis products.

-;In :order that those skilled in the art better may understand how the present invention can be carried into effect, the following examples are given by way of illustration and not by way of limitation. All parts are by weight.

Example i allyl isocyanurate in solution is in a molar ratio of 1 mole of the former in about 1.06 molesof the latter.

The solution is then distilled at atmospheric pressure. About 90% of the original amount of toluene is recovered, and a large proportion of the absorbed-hydrogen chloride is expelled. 'The residue, which still contains some absorbed ..(-or combined) hydrogen chloride, is slowly heated to 180 C. while the remainder of the toluene distills very slowly. At 180-220 C. the residue gives off a low-boiling product and-some gas (probably 1101) but, after a. few minutes, further heating at about 270-290 C. merely causes boiling of the unconverted triallyl isocyanurate. The distillate obtained by condensation of the vapors weighs parts and has a strong odor of allyl isocyanate. Upon analysis it is found that this solution contains 0.7 part of allyl isocyanate. This compound is identified by conversion to N-allyl-N-phenylurea that melts at l05-106 C. and which does not depress the melting point of a known sample of N-allyl-N'-phenylurea, M. P. l05-106 C.

Example 2 The apparatus employed in this example comprises an inner Pyrex glass tube, 5 mm. 0. D., leading almost to the bottom of a 36-inch x mm. 0. D. Pyrex glass tube filled to a depth of about 21 inches with triallyl isocyanurate. The dry hydrogen chloride gas is metered from a capillary flowmeter to the smaller or inner tube, and the pyrolysis products are passed from a side arm near the upper end of the larger tube into a "dry ice (solid carbon dioxide) trap-receiver.

The dry H01 gas is passed at a very low rate cc. per minute) through 51 grams of triallyl isocyanurate (21 depth) heated to about 240 C. After several hours the trap-receiver contains 3 grams of liquid having a strong lachrymatory action. It is identified as allyl isocyante by reaction with aniline and recrystallization of the resulting precipitate of N-allyl-N'-phenylurea.

Example 3 Same as in Example 2 with the exception that dry HCl gas at about 50 cc. per minute is passed through 50-55 grams of triallyl isocyanurate heated to 120-130 C. over a period of 7%,; hours. The dry ice trap-receiver contains 4 grams of brownish liquid having the characteristic odor of allyl isocyanate.

Example 4 Same as in Example 1 with the exception that about 0.106 mole of tri-n-butyl isocyanurate is substituted for 0.106 mole (26.5 parts) of triallyl isocyanurate. The product, n-butyl isocyanate, is identified by conversion to N-n-butyl-N- phenylurea and comparing the melting point of the 6 latter with the melting pointof a known sample of N-n-butyl-N'-phenylurea.

It will be understood, of course, by those skilled in the art that our invention is not limitedto the pyrolysis of the particular tri-(aliphatic hydro carbon) isocyanurates named in the above illustrative examples, since any other .isocyanurate of this class similarly can be pyrolyzed to yield varying amounts of the corresponding isocyanate. Examples of such starting isocyanurates have been given hereinbefore, and the names of others will be apparent to those skilled in the art from the general formula for such an isocyanurate as given earlier herein and from the definition of R as given with reference to that general formula; Likewise, the invention is not limited to the particular-conditions of pyrolysis described in the various" examples, since these conditions can be widely varied as will be clear from the general description of the invention as given in that DOI". tion of this specification prior to the examples.

The method of this invention is especially useful as a means of converting the less valuable and less useful tri-(aliphatic hydrocarbon) isocyanurates into more valuable aliphatic hydrocar bon isocyanates that have wider utility :in indus+ try than the isocyanurates from which they are produced. The method is relatively simple and inexpensive and, therefore, has obvious advantages over other methods of preparing isocyanates such as those involving phosgenation of the corresponding amine.

We claim:

1. The method of preparing an aliphatic hydrocarbon isocyanate represented by the formula RNCO, where R represents an aliphatic hydrocarbon grouping wherein any unsaturation is ethylenic unsaturation, said method comprising pyrolyzing the corresponding tri-(aliphatic hydrocarbon) isocyanurate under anhydrous con ditions in the presence of hydrogen chloride at a temperature of at least C. but below the temperature of decomposition of the isocyanate which is formed, and isolating the said isocyanate.

2. The method of preparing an alyky isocyanate which comprises pyrolyzing the corresponding trialkyl isocyanurate under anhydrous conditions in the presence of hydrogen chloride at a temperature of at least 120 C. but below the temperature of decomposition of the alkyl isocyanate which is formed, and recovering the said alkyl isocyanate.

3. The method of preparing an alkenyl isocyanate which comprises pyrolyzing the corresponding trialkenyl isocyanurate under anhydrous conditions in the presence of hydrogen chloride at a temperature of at least 120 C. but below the temperature of decomposition of the alkenyl isocyanate which is formed, and recovering the said alkenyl isocyanate..

4. The method of preparing allyl isocyanate which comprises pyrolyzing triallyl isocyanurate under anhydrous conditions in the presence of hydrogen chloride at a temperature within the range of C. to 270, and recovering the allyl isocyanate which is formed.

5. The method of preparing an aliphatic hydrocarbon isocyanate represented by the formula RNCO, where R represents an aliphatic hydrocarbon grouping wherein any unsaturation is ethylenic unsaturation, said method comprising distilling a solution of the corresponding tri- (aliphatic hydrocarbon) isocyanurate in an organic solvent which is inert to the said isocyanurate and its pyrolysis products, said solution contaming :ab'sorbed' :hydrogen chloride (and said distillation being continued until the saidsolverz't has rbeentremoved, heating "the residue contain ing absorbed hydrogen chloride at a temperature oft-etusleast- 12,0" 1C.' bi1t -.below the temperature of deefimposi'tionbf the isocyanate which-is formed, and'isolatin'g the" said isocyanate by' distillation from the heated residue.

6. The method-of preparing iallyl isocya'nate which-comprises distilling a solution of tria llyl isoeyanurate .an organic solvent which "is inert to thesaid isocyanura'te and its pyrolysis products, said solution containing-absorbed hy 'drogen ehloride-vand said distillation being continuedxnntibthe said solvent has been removed,

heating 1 the residue containing absorbed hydrogen chloride at a temperature within the range vof L80:- Cfito 2:! (S -and isolating allyl isocyanat-e by distillation from the heated residue and condensing lthe distillate.

73! e method 'of preparing 'allyl isocyanate o'mprises passing dry hydrogen vchloride gasinto-a solution of triallyl isocyanurate dis solved--1n dry-tolu'ene until the said solution has been saturated with hydrogen chloride, distilling theisaidfisoldti'on to remove the toluene, heating :izhe residue leon-taining tabsorbed hydrogen 'ohlonaemia *temperature within the range' 'of =180 to 220" and isolatingna'llyl i'socyanate' by distillation from ithe'i heated residue end condensing the distillate. I

8. The method of' fireparing an aliphatic hydroearbon .iisonyanate represented byeth formula RNCQ, where it :represnts an aaliphatie hydro- .carbon grouping "Ewherein zany unsaturation fis ejthyile'nic mnsaturationl -lsaid qme'thod comprising passingrdryiahydrogen ohloride gas .into the {dorrespondingstri daliphatie fihyd-roearbori) isocy'an urate while meatiiig the flatter-sat ts. 'rtemperature o'fvatsle'ast ;-l'-=. bht below the :temr'ierature :of de'doinposition (of the dsoey'anate i which is formed, iand--;recovering the said-isocyanate.

The method of preparing allyl isoeyanate whioh coniprisesapassing dry hydrogen chloride No-xeiiencscited. 

1. THE METHOD OF PREPARING AN ALIPHATIC HYDROCARBON ISOCYANATE REPRESENTED BY THE FORMULA RNCO, WHERE R REPRESENTS AN ALIKPHATIC HYDROCARBON GROUPING WHEREIN ANY UNSATURATION IS ETHYLENIC UNSATURATION, SAID METHOD COMPRISING PYROLYZING THE CORRESPONDING TRI-(ALIPHATIC HYDROCARBON) ISOCYANURATE UNDER ANHYDROUS CONDITIONS IN THE PRESENCE OF HYDROGEN CHLORIDE AT A TEMPERATURE OF AT LEAST 120* C. BUT BELOW THE TEMPERATURE OF DECOMPOSITION OF THE ISOCYANATE WHICH IS FORMED, AND ISOLATING THE SAID ISOCYANATE. 